An earthquake is a rapid shaking of the ground caused by the release of energy stored in the rocks of the Earth. When the hot, asphalt-like mantle changes and moves, so do the Earth's cool plates. The movement of Earth's plates can cause stress and strain on these rigid plates. This stress and strain can build up pressure in the layers of rock in the crust. This stress causes rocks to bend, squeeze, or break. When the stress and pressure are great enough, they are released in one quick event, an earthquake. Where several of these earthquakes occur, a crack develops. We call this crack a fault. (This may be a different meaning from the everyday meaning for which also has a special meaning in science.) In places where there are volcanoes, the shifting mantle causes earthquakes also. When the mantle moves, the ground shakes.
Where does the earthquake start?
The focus is the point under the earth where the earthquake starts and the energy is released. The epicentre is the point on the surface of the Earth directly above the focus. The epicentre of an earthquake is important. While the shock waves from an earthquake can be detected around the world, most of the damage occurs at the epicentre, or close to it. How does the force of the earthquake travel through the Earth?
Imagine that you have dropped a large rock in a bucket of water. You can see the vibrations in the rippling of the water on the surface. Imagine you dropped that large rock again but this time you put your hand on the bottom of the bucket before you dropped the rock. You can feel the vibrations of the rock hitting the surface of the water. You can feel the vibrations the rock makes because its energy travels through the water.
The energy from an earthquake moves in much the same way; it travels out from the centre in wave-like patterns called seismic waves. Some of these seismic waves travel on the surface, like the ripples you can saw in the water, and some travel through the Earth, like the vibrations you could feel on the bottom of the bucket. The surface waves can travel thousands of miles from the epicentre of an earthquake, and they can cause lots of damage. Like the waves that moved through the bucket of water, some of these waves travel through the Earth instead of on its surface. We call these body waves.
The seismic waves from the 1906 earthquake in San Francisco were felt some 9,100 miles (14642 km) away in Gottingen, Germany. Above is the record from a seismograph (an instrument used to record waves from earthquakes). The part of the record shown here spans about 26 minutes. Only body waves are shown in this record. When surface waves arrived later, the instrument went off-scale (that means the waves were larger than the seismograph could record). How is the strength of an earthquake measured? The newspaper headline read, �Strong 7.9 Quake leaves Peru reeling Arequipa hardest hit; at least 52 dead.� You can probably tell from the headline that this was a large and destructive earthquake. But, what does that 7.9 mean?
The 7.9 is the earthquake�s magnitude, a measure of how much energy an earthquake has released. In the United States we use the Richter scale to measure magnitude. The Richter scale ranges from 1 to 9 with 1 being small earthquakes and 9 being large earthquakes. On the Richter scale, earthquakes with a magnitude of 1 or 2 are small and cannot be felt by people. These small earthquakes are often not reported because they can be the result of a building being demolished as well as of a movement of the crust. Earthquakes with a magnitude of 3 or 4 are sometimes felt but are still considered small earthquakes. Earthquakes with medium magnitudes (4 to 6) are always felt and result in minor damage. Earthquakes with magnitudes larger than 6 are felt by everyone, and there is much damage and devastation. Size is not all that matters! In late 1811 and early 1812, New Madrid, Missouri was the site of a series of the largest and most geologically significant earthquakes in U.S. history. It is told that church bells tolled in Boston, and that the Mississippi River reversed flow and changed course. It is also told that fields were swallowed up, and new lakes were formed, all from this series of earthquakes. In 1812 this part of the country was scarcely populated or populated by people that did not read or write, so stories have been passed down through the generations and surely retold with a little more flavour. Though the personal devastation was not significant, the geological changes were.
What made these earthquakes shake so much more than many other earthquakes is the geology of the area. Thousands of years of sediment have built up in this region from glaciations and old ocean bottoms. Think of a plate of gelatine. If you shake the plate the top of the gelatine really shakes. The ground in New Madrid, with all the sediment, reacted to the earthquake much like the gelatine does when you shake it. An earthquake happening in these conditions causes more ground motion and therefore more devastation. In the San Francisco earthquake, people noticed the most devastation in parts of the city that were built on reclaimed land from the San Francisco Bay. The shaking of this soft sediment results in more damage.
Since the geological conditions of the area play a role in the destruction of an area hit by an earthquake, seismologists use a separate method to estimate the effects of an earthquake, called its intensity. Intensity should not be confused with magnitude. When scientists measure the intensity of an earthquake, they are trying to determine the effects of the quake at a particular place. The intensity, or amount of damage, is determined by observing the effects on the local people, the buildings, and the Earth�s surface.
To report intensity, seismologists use the Modified Mercalli Scale. The Mercalli Scale uses Roman numerals, I through XII, to establish levels of destruction, ground motion, and impact on humans.
The Mercalli Scale is not a precise measurement. It is based on observations people make after a quake. For example, if hanging objects swing back and forth, dishes and windows rattle, and there is slight damage, the earthquake may be rated IV on the Mercalli Scale. The intensity of an earthquake for an area depends on several factors. One of these factors is how far buildings are from the epicentre. Another factor is the type of ground under the buildings. If buildings are constructed on solid rock, they will have less damage than buildings built on less stable sediments.
Size and geology is not all that matters? Depth matters!
The effects of an earthquake also depend on how close to the surface the earthquake happens. This picture is from the San Francisco earthquake of 1906 and shows how the fence separated by about 8 feet. If earthquakes happen far below the surface, the effects are not felt. On August 19, 2002 near the Fiji Islands, two earthquakes with magnitudes greater than 7 happened just 7 minutes apart and only about 300 km (187 miles) from each other. These two very large and very close earthquakes almost went unnoticed because there was no damage, and they were only barely felt in nearby cities. These two earthquakes happened deep below the surface. The cool, brittle crust of the Earth is between 5 km and 70 km. These two earthquakes happened at 578 km (359 miles) and 674 km (419 miles) respectively below the Earth�s surface.
Once the ground starts shaking it will continue to shake in that region for a while. Some of this shaking is aftershocks. Aftershocks are earthquakes that follow the largest shock and are near the epicentre of the largest shock. When Alexander McAdie from the U.S. Weather Service described the aftershocks he experienced in San Francisco in 1906, he said, �They will come at greater intervals and grow weaker until they become absolutely imperceptible.�
Earthquakes can also lead to other earthquakes in the surrounding area. The shaking causes other pressure to be released and other earthquakes to occur. In New Madrid in 1812, the initial earthquake lead to other earthquakes in Ohio, Kentucky, and Tennessee.
When will the next earthquake hit? Today somewhere in the world there will be an earthquake. Many of these quakes are small and will not be felt by people.
How do scientists know where next big earthquake will be? They don't, but they are working on it. Geologists use instruments to measure changes in the Earth's surface and movement along faults. Scientists invented ground-based and satellite equipment with radar to make images of faults and keep track of movement. This equipment might detect small changes in the elevation of the land or a slight bend in the land. Scientists determine earthquake risk by monitoring active faults and studying faults where past earthquakes have occurred. This means that they are not collecting this data everywhere, and thus earthquakes often strike without warning.
Discussion Questions (before viewing site):
Where do most earthquakes happen in the world?
when and where was the last earthquake in the World?
How many earthquakes happened in Australia this week?
Earthquake!
Examine the following Website
An earthquake is a rapid shaking of the ground caused by the release of energy stored in the rocks of the Earth. When the hot, asphalt-like mantle changes and moves, so do the Earth's cool plates. The movement of Earth's plates can cause stress and strain on these rigid plates. This stress and strain can build up pressure in the layers of rock in the crust. This stress causes rocks to bend, squeeze, or break. When the stress and pressure are great enough, they are released in one quick event, an earthquake. Where several of these earthquakes occur, a crack develops. We call this crack a fault. (This may be a different meaning from the everyday meaning for which also has a special meaning in science.) In places where there are volcanoes, the shifting mantle causes earthquakes also. When the mantle moves, the ground shakes.
Where does the earthquake start?
The focus is the point under the earth where the earthquake starts and the energy is released. The epicentre is the point on the surface of the Earth directly above the focus. The epicentre of an earthquake is important. While the shock waves from an earthquake can be detected around the world, most of the damage occurs at the epicentre, or close to it.
How does the force of the earthquake travel through the Earth?
Imagine that you have dropped a large rock in a bucket of water. You can see the vibrations in the rippling of the water on the surface. Imagine you dropped that large rock again but this time you put your hand on the bottom of the bucket before you dropped the rock. You can feel the vibrations of the rock hitting the surface of the water. You can feel the vibrations the rock makes because its energy travels through the water.
The energy from an earthquake moves in much the same way; it travels out from the centre in wave-like patterns called seismic waves. Some of these seismic waves travel on the surface, like the ripples you can saw in the water, and some travel through the Earth, like the vibrations you could feel on the bottom of the bucket. The surface waves can travel thousands of miles from the epicentre of an earthquake, and they can cause lots of damage. Like the waves that moved through the bucket of water, some of these waves travel through the Earth instead of on its surface. We call these body waves.
The seismic waves from the 1906 earthquake in San Francisco were felt some 9,100 miles (14642 km) away in Gottingen, Germany. Above is the record from a seismograph (an instrument used to record waves from earthquakes). The part of the record shown here spans about 26 minutes. Only body waves are shown in this record. When surface waves arrived later, the instrument went off-scale (that means the waves were larger than the seismograph could record).
How is the strength of an earthquake measured?
The newspaper headline read, �Strong 7.9 Quake leaves Peru reeling Arequipa hardest hit; at least 52 dead.� You can probably tell from the headline that this was a large and destructive earthquake. But, what does that 7.9 mean?
The 7.9 is the earthquake�s magnitude, a measure of how much energy an earthquake has released. In the United States we use the Richter scale to measure magnitude. The Richter scale ranges from 1 to 9 with 1 being small earthquakes and 9 being large earthquakes. On the Richter scale, earthquakes with a magnitude of 1 or 2 are small and cannot be felt by people. These small earthquakes are often not reported because they can be the result of a building being demolished as well as of a movement of the crust. Earthquakes with a magnitude of 3 or 4 are sometimes felt but are still considered small earthquakes. Earthquakes with medium magnitudes (4 to 6) are always felt and result in minor damage. Earthquakes with magnitudes larger than 6 are felt by everyone, and there is much damage and devastation.
Size is not all that matters!
In late 1811 and early 1812, New Madrid, Missouri was the site of a series of the largest and most geologically significant earthquakes in U.S. history. It is told that church bells tolled in Boston, and that the Mississippi River reversed flow and changed course. It is also told that fields were swallowed up, and new lakes were formed, all from this series of earthquakes. In 1812 this part of the country was scarcely populated or populated by people that did not read or write, so stories have been passed down through the generations and surely retold with a little more flavour. Though the personal devastation was not significant, the geological changes were.
What made these earthquakes shake so much more than many other earthquakes is the geology of the area. Thousands of years of sediment have built up in this region from glaciations and old ocean bottoms. Think of a plate of gelatine. If you shake the plate the top of the gelatine really shakes. The ground in New Madrid, with all the sediment, reacted to the earthquake much like the gelatine does when you shake it. An earthquake happening in these conditions causes more ground motion and therefore more devastation. In the San Francisco earthquake, people noticed the most devastation in parts of the city that were built on reclaimed land from the San Francisco Bay. The shaking of this soft sediment results in more damage.
Since the geological conditions of the area play a role in the destruction of an area hit by an earthquake, seismologists use a separate method to estimate the effects of an earthquake, called its intensity. Intensity should not be confused with magnitude. When scientists measure the intensity of an earthquake, they are trying to determine the effects of the quake at a particular place. The intensity, or amount of damage, is determined by observing the effects on the local people, the buildings, and the Earth�s surface.
To report intensity, seismologists use the Modified Mercalli Scale. The Mercalli Scale uses Roman numerals, I through XII, to establish levels of destruction, ground motion, and impact on humans.
The Mercalli Scale is not a precise measurement. It is based on observations people make after a quake. For example, if hanging objects swing back and forth, dishes and windows rattle, and there is slight damage, the earthquake may be rated IV on the Mercalli Scale. The intensity of an earthquake for an area depends on several factors. One of these factors is how far buildings are from the epicentre. Another factor is the type of ground under the buildings. If buildings are constructed on solid rock, they will have less damage than buildings built on less stable sediments.
Size and geology is not all that matters? Depth matters!
The effects of an earthquake also depend on how close to the surface the earthquake happens. This picture is from the San Francisco earthquake of 1906 and shows how the fence separated by about 8 feet. If earthquakes happen far below the surface, the effects are not felt. On August 19, 2002 near the Fiji Islands, two earthquakes with magnitudes greater than 7 happened just 7 minutes apart and only about 300 km (187 miles) from each other. These two very large and very close earthquakes almost went unnoticed because there was no damage, and they were only barely felt in nearby cities. These two earthquakes happened deep below the surface. The cool, brittle crust of the Earth is between 5 km and 70 km. These two earthquakes happened at 578 km (359 miles) and 674 km (419 miles) respectively below the Earth�s surface.
Once the ground starts shaking it will continue to shake in that region for a while. Some of this shaking is aftershocks. Aftershocks are earthquakes that follow the largest shock and are near the epicentre of the largest shock. When Alexander McAdie from the U.S. Weather Service described the aftershocks he experienced in San Francisco in 1906, he said, �They will come at greater intervals and grow weaker until they become absolutely imperceptible.�
Earthquakes can also lead to other earthquakes in the surrounding area. The shaking causes other pressure to be released and other earthquakes to occur. In New Madrid in 1812, the initial earthquake lead to other earthquakes in Ohio, Kentucky, and Tennessee.
When will the next earthquake hit? Today somewhere in the world there will be an earthquake. Many of these quakes are small and will not be felt by people.
How do scientists know where next big earthquake will be? They don't, but they are working on it. Geologists use instruments to measure changes in the Earth's surface and movement along faults. Scientists invented ground-based and satellite equipment with radar to make images of faults and keep track of movement. This equipment might detect small changes in the elevation of the land or a slight bend in the land. Scientists determine earthquake risk by monitoring active faults and studying faults where past earthquakes have occurred. This means that they are not collecting this data everywhere, and thus earthquakes often strike without warning.
Discussion Questions (before viewing site):
Learning Object
Virtual Earthquake Lab - Find the epicentre of an earthquake